The invention relates to a method and an apparatus for compensating for luminance defects produced by the processing of the chrominance signals.
EP-A-0 176 093, DE-A-3 516 110 and EP-A-0 253 218 describe a method for compensating for receiver-end luminance defects caused by transmitter- end low-pass filtering of the gamma-predistorted chrominance signals, with the aid of correction signals. This is a method which is used purely at the transmitter end and is fully compatible with existing equipment. It leads to an improvement of the picture quality in the region of highly saturated colours.
In the modern picture coding methods MPEG-1 (ISO/IEC 11172) and MPEG-2 (ISO/IEC 13818), only the receiver end of the transmission path, the decoder, is standardized. The transmitter end of the transmission path, the encoder, is not standardized. It is only necessary to ensure that the encoder transmits a bit stream which can be decoded by a standard decoder. MPEG-2 is currently used in the transmission of digital television signals, a small number of encoders, but many decoders being required. When the costs for an MPEG-2 encoder permit a consumer application, e.g. digital MPEG-2 video recorders can be employed. For the purpose of compatibility, a standardized bit stream must be recorded on the storage medium (magnetic or optical medium) and the decoders fitted in these video recorders must comply with the standard. Competition between individual manufacturers can then be engendered by encoders which produce different picture qualities during picture reproduction with a standardized decoder.
The invention is based on the object of specifying, in the context of digital picture coding of signal components, a method for compensating for luminance defects produced by the processing of chrominance signals. This object is achieved by means of the method specified in claim 1. The invention is based on the further object of specifying an apparatus for application of the method according to the invention. This object is achieved by means of the apparatus specified in claim 6.
The invention has recognized that the abovementioned method for luminance correction can also advantageously be used in connection with modern picture coding methods such as e.g. MPEG-1 and MPEG-2. The principle behind this compensation method is the fact that as a result of the transmitter-end gradation or xcex3 predistortion, part of the luminance or brightness information contained in the picture is transmitted in the two chrominance signal components. This portion is greater, the smaller the luminance signal of the respective colour is and the greater the colour saturation thereof is. Each change in the chrominance signal components leads to a change in the luminance information transmitted by them. In the previous colour television transmission standards, such a change is, for example, the low-pass filtering of the two chrominance signal components prior to transmission. The coding of picture information according to the MPEG standards is also carried out with gradation-predistorted signals. The two chrominance signal components are changed by irrelevance reduction, which is performed by quantization of the DCT-coded signal values, and by the customary low-pass filtering. This change also effects a change in the luminance information contained in the chrominance signal components, which becomes visible during the reproduction of MPEG-coded video signals inter alia as blur in greatly saturated regions and as an interference signal in the luminance. This luminance interference signal is manifested like quantization noise.
The known method for luminance compensation in analogue television standards can now also be used advantageously in digital picture coding, e.g. according to the MPEG standards, in order to improve the picture quality. To that end, the chrominance is encoded and decoded again in the encoder. A correction signal is derived from the decoded chrominance signal and is used during the encoding of the luminance. The macroblocks which are motion-compensated for prediction are based on the correspondingly decoded chrominance signal and on the decoded, corrected luminance signal.
In principle, the method according to the invention consists in executing the following steps for the purpose of compensating for luminance defects produced by the processing of chrominance signals:
encoding of the chrominance signal components for a pixel block;
derived from the encoded chrominance signal components, encoder-end simulation of the encoded chrominance signal components of the pixel block which are decoded by a receiver;
formation of a luminance correction signal with the aid of the simulated chrominance signal components and the as yet unencoded luminance signal component;
encoding of the luminance signal component, combined with the luminance correction signal, for the respective pixel block.
Advantageous developments of the method according to the invention emerge from the associated dependent claims.
In principle, the inventive apparatus for compensating for luminance defects produced by the processing of chrominance signals is provided with:
first encoding means for the chrominance signal components from a pixel block;
means which are fed with the encoded chrominance signal components and serve for the encoder-end simulation of the encoded chrominance signal components of the pixel block which are decoded by a receiver;
correction means for forming a luminance correction signal with the aid of the simulated chrominance signal components and the as yet unencoded luminance signal component;
second encoding means for the luminance signal component combined with the luminance correction signal, for the respective pixel block.
Advantageous developments of the apparatus according to the invention emerge from the associated dependent claims.